The temperature dependence of the spin relaxation of coupled polaron pairs in organic semiconductors

• The molecular vibrations induce the spin relaxation of polaron pair.
• The pair relaxes to a spin state with 1/2 in singlet state and 1/6 in each triplet state.
• The relaxation time is inversely proportional to the temperature’s square.
• The connection to experimental data on magneto-electroluminescence is discussed.

The coupled polaron pair distributed in adjacent organic molecules has spin relaxation due to molecular vibration. We construct a Hamiltonian with on-site energy fluctuation to investigate its temperature dependence. The relaxation occurs between spin singlet and one of the spin triplet states, but not between spin triplet states. It is found that the polaron pair finally relaxes to a spin state with half probability in spin singlet state and 1/6 probability in each of the spin triplet states, and the relaxation time is inversely proportional to the temperature’s square. The connection to the magneto-electroluminescence experiment is also discussed.

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